The long-term objective of this research is to utilize the incorporation of selenium or tellurium, either externally (through the association of Se or Te containing substrates, inhibitors, or coenzymes) or internally (via in vivo protein biosynthesis using seleno- or telluromethionine or seleno- or tellurotryptophan), as a means of obtaining heavy-atom protein derivatives whose investigation will readily lead to (1) solution of the phase problem and elucidation of three-dimensional structure and (2) the investigation of protein function via 77Se or 125Te NMR spectroscopy. Recent progress in protein isolation and purification methods together with improvements of crystallization techniques and multinuclear high-field NMR technology has led to an exponential growth in the number of biomedical problems which are approached using macromolecular crystallography and NMR spectroscopy. Three methods of incorporation of selenium or tellurium into proteins are envisioned. First, in a biosynthetic manner, we propose to employ in vivo protein biosynthesis to incorporate seleno- or telluroamino acids in place of their usual counterparts. Second, we propose incorporation through the association of selenium- or tellurium-containing cofactors with proteins. Third, we propose the incorporation of other selenium- or tellurium-containing compounds including inhibitors, transition state analogs, affinity reagents and allosteric effectors. We will then determine the stoichiometry and site(s) of selenium or tellurium incorporation into the protein or, where pertinent, measure the binding constant for the interaction of the selenium or tellurium-containing ligand with the protein. The seleno- or telluroproteins (as apo- or haloproteins) will then be investigated by x-ray crystallography and high field 7Se or 125Te NMR spectroscopy. At the present time, the two least routine and usually critical stretches on the road to a successful macromolecular structure determination are (1) growing suitable crystals and (2) obtaining isomorphous heavy atom derivatives. The proposed research involving the incorporation of tellurium compounds can significantly strengthen the arsenal of prospective heavy atom reagents available to protein crystallographers and, as a consequence, shorten and facilitate determinations of the three-dimensional structures of many biomedically important macromolecules.